1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to improvements in or to an ignition timing control system, which are intended to prevent knocking irrespective of the octane value of fuel used in the engine.
2. Prior Art
Generally, if a fuel having a low octane value is used in an internal combustion engine which is designed to operate on a fuel having a high octane value, knocking can occur. To overcome this problem, an ignition timing control system for an internal combustion engine has been proposed by the assignee of the present application by Japanese Provisional Patent Publication (Kokai) No. 2-213190), which comprises operating condition detecting means for detecting operating conditions of the engine, knocking detecting means for detecting knocking occurring in the engine, basic ignition timing determining means for determining basic ignition timing based upon operating conditions of the engine, ignition timing correcting means for correcting the basic ignition timing by a correction value which is determined in response to an output from the knocking detecting means, and advancing correction inhibiting means for inhibiting correction of the correction value toward an advanced side with respect to a predetermined value in a timing retarding direction which is determined based upon operating conditions of the engine, when the correction value is greater than the predetermined value.
According to this proposed system, when knocking is detected, the ignition timing is corrected in the retarding direction. When the correction value in the retarding direction exceeds the predetermined value, it is decided that a fuel having a low octane value is used in the engine, and then correction of the correction value toward an advanced side with respect to the predetermined value is inhibited.
However, in the case where there occurs a change in the engine load, the proposed system has the following disadvantage:
FIG. 13(A) shows the relationship between engine load and ignition timing (timing advance) .theta.ig. In the figure, symbol .theta.MAP represents basic ignition timing which contains no retarding correction, and the difference (.theta.MAP-.theta.LMT) corresponds to the above-mentioned predetermined value. The region below the line .theta.LMT corresponds to ignition timing applied when a low octane fuel is used. Let it be assumed that a retarding correction amount .theta.KNOCK is applied as .theta.KNOCK1 when the engine load assumes a value L1 (point P1). The correct amount .theta.KNOCK1 is greater than the predetermined value (.theta.MAP-.theta.LMT), and therefore it is decided that a low octane fuel is used. If the engine load then abruptly increases up to L2, the retarding correction amount .theta.KNOCK still remains .theta.KNOCK1 (point P2) immediately after the abrupt increase, resulting in occurrence of knocking. Accordingly, the retarding correction amount .theta.KNOCK is gradually corrected toward an increased amount .theta.KNOCK2 at which no knocking can occur (P3), by repeatedly adding a retard unit amount DKNOCK to the amount .theta.KNOCK. Then, if the engine load suddenly decreases from L2 to L1, the correction amount .theta.KNOCK2 is still applied immediately after the sudden decrease (P4), which results in overcorrection and hence an excessive drop in the engine output. If the engine load thereafter remains at L1, the retarding correction amount .theta.KNOCK is gradually corrected to .theta.KNOCK1 by being decremented by an advance unit amount DADV.
As mentioned above, the proposed system has the disadvantage of re-occurrence of knocking or excessive drop in the engine output immediately after an abrupt change in the engine load.